This invention relates to a novel electrophotographic photosensitive device, and more particularly to an electrophotographic photosensitive device having a surface protective layer of low density of localized states on the surface of a photoconductive layer, which is suitable for a laser beam printer, a copying machine, etc.
The electrophotographic photosensitive device, which the present invention is directed to, has hydrogen-containing amorphous silicon, or organic photoconductor as a material for the photoconductive layer. The present invention is particularly suitable to an electrophotographic photosensitive device using hydrogen-containing amorphous silicon as a photoconductive material.
Some of the electrophotograhic photosensitive devices use hydrogen-containing amorphous silicon or selenium or organic photoconductor as a material for the photoconductive layer.
Different from the electrophotographic photosensitive device using selenium as a material for the photoconductive layer, the electrophotographic photosensitive device using hydrogen-containing amorphous silicon as a material for the photoconductive layer has no toxicity and is easy to handle. It is also equivalent with respect to the photosensitivity, photo response, dark resistance, etc., to the electrophotographic photosensitive device using selenium as a material for the photoconductive layer. Furthermore, the hydrogen-containing silicon has a higher hardness than that of selenium, and thus an electrophotographic photosensitive device with a long life can be expected. However, it has a poor moisture resistance and a poor corona resistance and is also more susceptible to light deterioration. Thus, a satisfactory electrophotographic photosensitive device with a long life has not been obtained yet.
In an electrophotographic process applicable to a laser beam printer or a copying machine, on the other hand, the surface electric charge is made to be scattered by a carrier generated by light exposure, after the surface of the photosensitive device has been kept at a high potential, and thus the photosensitive device must take a structure of high electric resistance so as to keep a substantial surface potential. However, a hydrogen-containing amorphous silicon prepared by glow discharge can have a dark resistance as high as only 10.sup.9 -10.sup.10 .OMEGA..multidot.cm and cannot have a higher resistance. To overcome the disadvantage, an electrophotographic photosensitive device using carbon, nitrogen or oxygen-containing amorphous silicon to increase the resistance as a surface layer has been disclosed [e.g. Japanese Patent Application Kokai (Laid-open) No. 54-145,537]. However, it has been found that the carbon, nitrogen or oxygen-containing amorphous silicon film with a higher resistance is liable to undergo deterioration like the hydrogen-containing amorphous silicon. Furthermore, the carbon, nitrogen or oxygen-containing amorphous silicon has a poor adhesion to the hydrogen-containing amorphous silicon, and thus can be easily peeled off.